Experimental and molecular dynamics study on the reaction characteristics of methane-ammonia mixed fuel on SOFC anodes

Abstract

This study employs experiment and molecular dynamics (MD) simulations to investigate the reaction characteristics of methane-ammonia mixed fuels on a Ni-yttria-stabilized zirconia (YSZ) anode of solid oxide fuel cells (SOFCs). Experimental results found that when a methane-ammonia mixed fuel is supplied, the ammonia decomposition reaction occurs preferentially, and the steam reforming of methane is inhibited. However, at a higher temperature, the decomposition rate of ammonia decreases with a lower mole fraction of ammonia, and the decline in the methane reforming rate becomes gentler. MD simulations reveal that water adsorbs onto YSZ through electrostatic interactions, forming stable multilayer adsorption. In contrast, ammonia adsorbs onto YSZ through interfacial bonding, resulting in monolayer adsorption that is more susceptible to temperature. At higher temperatures, lower ammonia mole fraction and higher water mole fraction significantly reduce the adsorption of ammonia on YSZ. This reduction limits the supply of ammonia from YSZ to the Ni surface, consequently decreasing the ammonia decomposition rate and hence lessening the inhibitory effect of ammonia on steam methane reforming. It is also found that, ammonia molecules exhibit greater surface diffusion than water, enabling more effective mobility from YSZ to the Ni surface where they occupy reaction sites, resulting in the inhibition of the steam methane reforming reaction. In addition, the extensive adsorption of water molecules can significantly enhance interfacial heat transfer, which may enhance the endothermic methane steam reforming reaction.